1. Field of the Invention
The present invention relates to a magnetic resonance imaging system for performing magnetic resonance imaging by a three-dimensional Fourier transform (3 DFT) method.
2. Description of the Related Art
Magnetic resonance (MR) phenomenon is a phenomenon in which a specific atomic nucleus which has a non-zero spin and a magnetic moment generated by the spin and is placed in a static field resonantly absorbs only an electromagnetic wave having a specific frequency. This atomic nucleus resonates at an angular frequency .omega..sub.O (.omega..sub.O =2.pi..gamma..sub.O ; .gamma..sub.O is a Larmor frequency) represented by the following equation: EQU .omega..sub.O =.gamma.H.sub.O
where .gamma. is a gyromagnetic ratio unique to the type of an atomic nucleus, and H.sub.O is a static field intensity.
In a magnetic resonance apparatus for examining and diagnosing a living body by using the MR phenomenon, an MR signal (e.g., an MR echo signal or an FID (free induction decay) signal), which is an electromagnetic wave having a frequency equivalent to the specific frequency induced by the above magnetic absorption, is detected and is subjected to signal processing, thereby obtaining diagnostic data (e.g., a slice image of a patient) reflecting an atomic nucleus density, a longitudinal relaxation time T.sub.1, a transverse relaxation time T.sub.2, the flow of a body fluid (e.g., a blood), and chemical shift data. Such an apparatus allows noninvasive acquisition of various data in an object to be examined, and hence is very effective for medical diagnosis.
In acquisition of diagnostic data by MR, MR can be theoretically excited at the entire portion of an object to be examined which is placed in a static field so as to acquire signals generated by the MR. In a practical apparatus, however, MR excitation and signal acquisition are performed with respect to a specific portion due to limitations on the arrangement of the apparatus or clinical requirements for diagnosis images.
As a typical example of such a magnetic resonance apparatus' a magnetic resonance imaging (MRI) system which is mainly designed for acquisition of MR image data, i.e., distribution data of the above-described various MR data is known.
The MRI system mainly comprises: a static field coil system for generating a homogeneous static magnetic field in an imaging region in which an object to be examined is to be placed; a gradient field coil system for generating gradient fields whose intensities are gradually and linearly changed in predetermined directions in the imaging region; a probe including a transmission/reception coil system for transmitting a high-frequency rotating magnetic field (high-frequency pulse) to the imaging region, and detecting an MR signal (e.g., an MR echo signal) induced by MR; a static field control system for performing energization control of the static field coil system; a transmitter and a receiver for respectively transmitting a high-frequency field and receiving an MR signal through the probe; X, Y, and Z gradient power supplies for respectively causing said gradient field coil system to generate gradient fields in orthogonal X-, Y-, and Z-axis directions; a sequencer for controlling the transmitter, the X, Y, and Z gradient power supplies in accordance with an image data acquisition sequence based on a predetermined imaging method such as a Fourier transform method; a computer system for controlling the sequencer and performing signal processing of an MR signal detected by the receiver; and a display apparatus for displaying data obtained by the signal processing as an image.
Imaging is performed in the following procedure. An object to be examined is placed in the imaging region. While a static field is generated in the imaging region by the static field control system and the static field coil system, the sequencer is operated, and a predetermined pulse sequence, e.g., a pulse sequence based on a spin-echo method, for acquiring MR data necessary for imaging is executed.
In accordance with the pulse sequence, the transmitter is driven to cause a coil system of the probe to apply a pulse-like high-frequency rotating magnetic field, i.e., a high-frequency pulse (typically, a selective excitation pulse or nonselective excitation pulse having a flip angle of 90.degree. and/or 180.degree.) to the object in the imaging region, and the X, Y, and Z gradient power supplies are driven to cause the gradient field coil system to apply X, Y, and Z gradient fields Gx, Gy, and Gz to the object in the imaging region as a slice gradient field (Gs), an encode gradient field (Ge), and a readout gradient field (Gr), respectively. MR is excited at a slice portion having a thickness determined by the selective excitation pulse and the slice gradient field which is applied while the selective excitation pulse is applied to the object. An MR signal from the slice portion is then acquired by the coils of the probe, and data corresponding to one line in an image region in a Fourier space is obtained. In order to generate MR signals corresponding to one frame, such a sequence is normally repeated a predetermined number of times to obtain MR data. MR data obtained in each sequence is subjected to reconstruction processing to generate and display, e.g., a two-dimensional MR image.
The above-description is associated with imaging based on the two-dimensional transform method. In addition to the two-dimensional Fourier transform method, a three-dimensional transform method used for construction of a three-dimensional image is available as an imaging method.
Imaging based on a known three-dimensional transform method including selective excitation will be described below with reference to FIGS. 1 and 2A to 2C.
FIG. 1 shows a pulse sequence of the three-dimensional transform method including selective excitation in which a gradient field echo method is employed as a method of generating MR echoes. FIG. 2A shows a selective excitation region V as an object to be imaged in order to explain excitation processing. FIG. 2B shows an acquired data group, i.e., three-dimensional volume data VD in order to explain data acquisition processing. FIG. 2C shows a three-dimensional image VI obtained by performing reconstruction processing of the three-dimensional data VD by the three-dimensional Fourier transform method in order to explain reconstruction processing.
In a period I shown in FIG. 1, while, a slice gradient field Gs, e.g., a gradient field Gz in the Z-axis direction is applied to an object P to be examined, which is placed in an imaging region, in the form of a pulse, a selective excitation pulse as an RF (radio frequency) is applied to the object P. A selective excitation region V of the object P is excited by the selective excitation pulse and the slice gradient field Gs, and MR occurs at the region V. The selective excitation region V has a size Wex determined in the slice direction, i.e., the Z direction by the selective excitation pulse and the slice gradient field Gs. The selective excitation region is called a slab. The slab consists of a plurality of slices. That is, a slice region based on the RF pulse and the slice gradient field Gs is called a "slab". This slab is decomposed into a plurality of slice regions by reconstruction processing. Each slice region is called a "slice". (It is apparent that application of the slice gradient field Gs includes inverse application for compensation.)
Subsequently, in a period II shown in FIG. 1, a slice gradient field Gs (e.g., a Z-axis gradient field Gz) and an encode gradient field Ge (e.g., a Y-axis gradient field Gy) for encoding a phase in two directions, and a readout gradient field Gr (e.g., an X-axis gradient field Gx) for generation of echoes are applied in the form of a pulse. The polarities of the read gradient field Gr (X-axis gradient field Gx) are then reversed and the field Gr is applied in the form of a pulse having the opposite polarity. During this application period, an MR echo signal is acquired. The MR echo signal induced by the inversion of the read gradient field Gr (X-axis gradient field Gx) is encoded by the slice gradient field Gs (X-axis gradient field Gz) in the slice direction (Z-axis direction) and is also encoded by the encode gradient field Ge (Y-axis gradient field Gy) in the encode direction (Y-axis direction). As a result, position data of the MR echo signal in the respective directions are phase-encoded. The three-dimensional data DV (corresponding to the slab) shown in FIG. 2B can be obtained by repeating the above-described sequence a number of times corresponding to the matrix size of MR image data to be reconstructed while sequentially changing the intensity of each gradient field for each encoding.
A slice thickness (or slice depth) w is determined by the degree of encoding in the slice direction. A slice count N is determined by the number of times of encoding in the slice direction. That is, the relationship between a slab thickness (or slab depth) W and slices can be represented by W=N.multidot.w.
Note that the thickness of an actual excitation region determined by the intensities of a selective excitation pulse and the slice gradient field Gs superposed thereon is called a slab excitation thickness Wex.
Selective excitation characteristics will now be considered. Selective excitation characteristics (i.e., slice characteristics) are equivalent to distribution characteristics of MR signal levels in relation to displacement of a gradient field applied together with an selective excitation pulse. Ideally, a perfect rectangular characteristic curve (i.e., characteristics in which, in the slice direction, MR signal levels outside a slab are zero, MR signal levels at boundaries of the slab steeply rise, and MR signals having a uniform level are obtained within the slab) is obtained. Actual selective excitation characteristics, however, do not exhibit a perfect rectangular shape. Therefore, the slab excitation width Wex is generally defined by a half width in a curve obtained by respectively plotting displacement and signal levels on the abscissa and the ordinate, as shown in FIG. 3 (showing selective excitation characteristics commonly obtained by the spin-echo method).
As described above, since imperfect selective excitation characteristics are obtained, the following problems are posed in MRI by the conventional three-dimensional Fourier transform method including selective excitation.
If an ideal (rectangular) selective excitation characteristic curve is obtained, N slice images can be obtained by setting the slab excitation thickness Wex=slab thickness W, as shown in FIG. 4. In practice, however, ideal selective characteristics cannot be obtained as described above, the signal levels of the end slices of the slab V are lowered as shown in FIG. 5. Hence, a good image cannot be obtained. In addition, as indicated by dotted lines in FIG. 5, the MR signals at both the ends of the slab excitation thickness Wex involve signals from other end portions due to the influences of aliasing. As a result, at both ends of an image of the slab V shown in FIG. 6A, artifacts (indicated by dotted lines) based on the data of other ends are produced, as shown in FIG. 6A and 6B.
The above-described phenomenon occurs even in the spin-echo (SE) method in which dispersed spin phases are focused by a nonselective excitation pulse (typically, a 90.degree.-180.degree. pulse train is used), and in the gradient field echo (FE) method in which spin phases are focused by inverting a readout gradient field instead of the application of a nonselective excitation pulse, and the MR data acquisition time can be shortened by a time required for the application of a nonselective excitation pulse. Especially, in the FE method, the above phenomenon is further complicated due to the following reasons.
In the FE method, scan parameters are a pulse repetition time T.sub.R, an echo time T.sub.E, and a flip angle .alpha.. In this case, a three-dimensional Fourier transform method (including selective excitation) to which the FE method is applied will be described.
If the pulse repetition time T.sub.R is fixed, and the flip angle .alpha. is changed, an obtained signal intensity is changed as shown in FIG. 7.
A signal intensity S is represented as follows: ##EQU1## A flip angle .alpha..sub.O at the maximum signal intensity (normally called an Ernst angle) can be represented as follows: ##EQU2## The flip angles .alpha. at both the ends of a selectively excited region, i.e., a slab become smaller than a set value due to incomplete selective excitation characteristics. Therefore, if T.sub.R &gt;&gt;0, .alpha..sub.O .perspectiveto..pi./2 (=90.degree.). The obtained selective excitation characteristics are relatively close to the ideal rectangular shape, as shown in FIG. 8.
If, however, T.sub.R becomes smaller (shortened), .alpha..sub.O is reduced. In this case, if .alpha.&lt;.alpha..sub.O as shown in FIG. 9A, both the end portions of the curve, whose signal levels are not flat, are broadened. If .alpha..perspectiveto..alpha..sub.O as shown in FIG. 9B, the central flat portion is widened. If .alpha.&gt;.alpha..sub.O as shown in FIG. 9C, the signal levels of both the end portions are increased in signal level as compared with the central flat portion.